Linearization of RF Power Amplifiers

Abstract

Linearization of RF power amplifiers is surveyed, reviewed and analyzed. Cartesian feedback is specifically presented as an effective means of linearizing an efficient yet non-linear power amplifier. This reduces amplifier distortion to acceptable levels and enables the transmission of RF signals utilizing spectrally efficient linear modulation schemes with a lower consumption of DC power. Results from constructing experimental hardware shows an intermodulation distortion (IMD) reduction of 44dB (achieving a level of â��62dBc) combined with an efficiency of 42% when transmitting Ã°/4 QPSK. The careful amplifier characterization measurement method presented predicts performance to within 2dB (IMD) and 4% (efficiency) of practical measurements when used in simulations. A comprehensive stability analysis is developed using piecewise amplifier models within a multiple-input, multiple-output block diagram representation of the cartesian feedback loop. The analysis shows how RF amplifier non-linearity, the RF phase adjuster setting, loop gain, bandwidth and delay affect stability. A graphical interpretation of the analysis is given that indicates how stable a given RF amplifier will be when setting up a practical cartesian feedback loop. Instability is shown to result when the amount of RF phase rotation introduced by AM/PM distortion, and by setting error in the RF phase adjuster within the loop, equals the open-loop phase margin. For one of the amplifiers investigated, the analysis predicts that instability results just after the transistor turn-on region when the phase adjuster is adjusted above optimum, and instability also results at transistor saturation when adjusted lower than optimum. This is also demonstrated with experimental hardware. From the analysis, the perturbated behaviour of the non-linear piecewise amplifier model is shown to display two forms of operation when placed in a feedback loop, namely: spiral mode and stationary mode. Spiralling tends to cause the noise floor of the output spectrum to rise on one side depending on the direction of the spiral. The direction is in turn dependent on the setting of the RF phase adjuster within the loop. When the phase adjuster is in the forward path, phase adjustments lower than optimum, will cause the noise to rise on the right side of the output spectrum (anti-clockwise spiralling) and viceversa. With the phase adjuster in the feedback path the reverse is true. Loops with low stability margins are demonstrated to exhibit closed-loop peaking which can affect the out of band noise performance of a cartesian feedback transmitter. In order to achieve a non-peaking condition for a first order loop with delay, the phase margin of the loop needs to be around 60Â°. It is also possible to approximately predict the degree of peaking from the gain and phase margins. Further investigation of noise performance suggests the loop compensation should be placed as far up the forward chain as possible (i.e. close to the power amplifier) in order to minimize the out-of-band noise floor. This too is demonstrated experimentally. The concept of dynamic bias is also presented as a method to improve cartesian feedback efficiency. The method works by setting up optimum bias conditions for the power amplifier (derived from amplifier characterizations) and then having the cartesian feedback loop make fine adjustments to the RF drive to achieve the exact required output. This way the bias conditions do not have to be applied perfectly, implying simple (i.e low switching frequency) switched mode power supplies can be used to apply the desired collector voltage for example. The simple step-down switch mode power supply constructed achieved an efficiency of 95% at high output levels. Applying it to a cartesian feedback loop markedly improved efficiency. At an output power of 20dBm average, the linearized amplifier efficiency lifted from 45% to 67%, an improvement of over 20% and a reduction in current consumption by 33%.